Contribution of research borehole data to modelling fine-grained turbidite reservoir analogues,Permian Tanqua-Karoo basin-floor fans (South Africa)

摘要

Outcrop analogue studies can be augmented and constrained by drilling research wells through the same stratigraphic interval.Close-to-outcrop wells help to validate outcrop observations with well log and core data and thus improve the use of such data in actual field developments.Research wells located further away from the outcrops increase the spatial data coverage and can give important insight into regional facies distributions and net:gross changes.In the Tanqua-Karoo Basin (South Africa),seven wells were drilled into fine-grained sand-rich basin-floor fans and interfan mudstones to supplement outcrop data.Three close-to-outcrop wells proved useful in establishing characteristic log responses of the main architectural elements identified from the nearby outcrops.Lithofacies were correcdy identified in more than 80% of cases using an artificial neural network.Borehole images provided detailed information on sedimentary structures,including a wealth of palaeocurrent data from climbing ripples that significantly enhanced the interpretations based only on outcrops.Wells sited away from the outcrops supplied information on lateral thickness and facies trends and intrafan stacking patterns,which helped to define the stratigraphic evolution of the fans.The combined data indicate that deposition was controlled in part by subtle basin-floor topography,and that intrafan lobe switching took place,leading to internal subdivisions that potentially caused effective compartmentalization of the basin-floor fan.